If you want to get rid of chemical weapons, the best way to do it is to work with Assad. He presented this package with Russia of getting rid of the weapons because he thought it would re-legitimize him. If America is willing to come to terms with Assad's surviving in the parts of the country that he already owns, I think Assad will be happy to deliver those chemical weapons, which have not been an important part. He wants to use them to help him win stability and his own longevity in the areas he controls. If America is willing to help him survive there, he will give up his chemical weapons, and Russia will help build him up, and he will be a happy guy.
I think Assad is coming to the conclusion that he cannot re-conquer the whole country. I think Russia and Iran would be happy to have Assad survive in Damascus, because this would preserve all the national interest that they need, which is to be able to resupply Hezbollah, the Shiites in Lebanon, and for the Russians to have a toehold in the Arab-Israeli conflict and to have a port on the coast and be a player in the Middle East.
Regime change is going to bring the end of those things to all the actors. They are not going to cooperate with the United States, whether on chemical weapons, humanitarian issues, or ceasefire, as long as they believe that the United States is trying to undermine their interests.
If you want to get rid of chemical weapons, you have to deal with Assad. That's why he presented the package in the first place: he did not want to be destroyed.